Variable resistance device



June 18, 1935. L CREAGER 2,005,456

VARIABLE RESISTANCE DEVICE Filed June 11. 1932 2A 5 HF sAunflLAn-n,

1* I I''. I r

INVENTOR Freda r-'Lc k LC reager,

HIS A TORNEY Patented June 18, 1935 UNITED STATES PATENT OFFICE VARIABLE RESISTANCE DEVICE poration of Delaware Application June 11, 1932, Serial No. 616,629

' 9 Claims.

My invention relates to variable resistance devices and it has particular relation to volume control rheostats and potentiometers of the wire wound type commonly used in radio receiving apparatus.

In the construction of wire wound, variable contact, potentiometers and rheostats for radio apparatus, with which I am familiar, it has been the practice for certain volume control purposes, to provide a taper, i.e. to alter the rate of variation of resistance per degree of movement of the contact. Among the several present known methods or means for tapering variable resistors of the wire wound type, the following are well known: Electro-plating of low resistance metal on selected portions of the turns of wire of a resistor element; use of two or more wires, of different specific coefficients of resistance, in series, to obtain the desired resistance gradient; use of an auxiliary winding of different pitch in contact with the wire turns at the end of the element to be tapered; variation of the spacing between turns; use of an insulating support having different areas of cross-section at selected portions throughout its length; etc.

'These, as well as several other less known tapering means, have been investigated, but it has been found that they are unsuitable for highquality, low-cost production. Forming the taper by electro-plating, above mentioned, involves the use of a special clamping electrode in an electrolytic bath, washing operations, etc., and generally such a process has been relatively expensive and difficult to control, requiring highly skilled operators. Some of the other above processes have required special operations during the machine winding process, e. g'. stopping the machine to anchor or splice ends of turns. Another process included above, involves feeding the wire to a winding form at different speeds.

It is, accordingly, a principal object of my invention to provide a new and improved means of constructing the resistance element of variable resistor devices of the moving contact type, whereby the rate of change of resistance per degree of movement of the moving contact may be varied to suit the particular purpose for which the device is intended.

My invention has particular application to rheostats and potentiometers such as are used in radio receivers wherein the resistance element or conductor is in an annular form and the resistance is increased or decreased by means of an angularly 'movable contact arm. In general, the sound output fromra radio receiver is controlled by means of a variable resistance in some part of the circuit, and the well known wire wound rheostat is often employed. A disadvantage in the use of the ordinary wire wound rheostat is that it does not allow a control of 5 sound volume which is uniform throughout the range of the rheostat. This is due to the fact that the resistance of the rheostat is ordinarily a linear function of the angle of movement of the contact arm, whereas the sound output is not always a linear function of the resistance.- It is, therefore, desirable to use a rheostat, the resistance variation of which is other than a linear function of the movement of the contact arm so that the variation in sound volume may be substantially proportional to the movement of the contact arm.

In carrying out my invention I have provided a potential regulating or volume control device utilizing an elongated resistor element, of which a selected length, corresponding to the low volume region of adjustments, is appreciably reduced in resistance by the use of a plurality of shunting or short-circuiting means in contact therewith and spaced apart a predetermined distance.

Further in accordance with my invention I have provided an electrically tapered variable resistor of the wire wound type, the nature of the tapering construction of which is such that the resistor element can be simply wound with uniformly spaced wires upon an insulating form of ordinary shape, the tapering being effected by treatment of a selected portion of said form prior to winding, as by depositing, spraying, or paint- 35 ing areas of relatively low resistance material of predetermined size and spacing on the surface of said form.

More specifically, in accordance with my invention, I have provided a tapered variable resistor device of the wire wound type having a movable contact element, wherein a high resistance conductor is helically wound on an insulating surface of a form, the surface thereof being interrupted at selected points by sprayed low resistance surface areas with which the conductor makes Contact for the purpose of altering the resistance per unit length of the resistor element.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment, when read in connection with the accompanying drawing, in which:

Figure 1 is a view in perspective of a resistor strip or form and a mask for carrying out my invention;

Fig. 2 is a front view of the resistor form and mask of Fig. 1 in assembled relation, ready for the taper forming operation; I

Fig. 2A is an enlarged end view in section lines 2A-2A of Fig. 2, serving to illustrate more clearly, an operation in forming a tapered resistor in accordance with my invention;

Fig. 3 is a front view of the resistor structure, serving to illustrate a. further development in my process of forming a taper;

Fig. 3A is an enlarged end view in section, taken on lines 3A-3A of Fig. 3; and

Fig. 4 is a view in' perspective of a circularly formed resistor element, embodying my invention, showing the relation of the essential parts for assembly.

Referring to the drawing, an insulating form or strip 5 preferably of flexible bakelite, having been threaded along the edges 1 and l for receiving turns of resistance wire, is provided with;

a mask 9 of soft gummed paper or the like. This mask is provided with a desired number of suitably shaped and spaced apertures II and 2 located at the points at which it is desired to form coatings or areas of metal l4 and I5 upon the insulating form 5 (Fig. 3). Prior to the metal coating process, the surface portions of the form 5 exposed through perforations I l and I 2 of the mask, are roughened, preferably by sand blasting with fine sand or emery, thereby removing the surface glaze or resinous substanceon the form in order to prepare the latter for proper adhesion of the metal coating.

After sand blasting, the structure is blown clear of dust by means of an air jet. The form, with the mask still in place, is then subjected to a vaporous stream of metal as shown at I! in Fig. 2A in order to form the metal coats on the exposed areas by a process known as the Schoop metal vaporizing process. The metal is prefably of tin or similar malleable substantially nonoxidizing material and may be applied to both sides and edge of the top of the strip, as shown at IS in Fig. 3A. However, I have found that it is sufficient to simply deposit the shunt coats upon one side of the strip, extending it preferably at least half way over the edge, as with the spray gun l9 fixed in a position at a 45 deg. angle with respect to the side of the strip, as shown in Fig. 2A. In this case a succession of strips may be moved on a carriage past the spray gun. It is highly desirable that at least a portion of the edge be coated in order to insure firm contact with the turns of wire. It has been found practiclal to deposit the shunt areas along the edge on y.

The mask is next stripped from the form, leaving clean-cut, clearly defined metal surface areas It and I5 (Fig. 3), forming relatively low resistance shunts for the high resistance conductor, which areas are securely attached to the insulating strip. If desired, the mask may be removed from the insulating form after sand blasting and prior to the metal coating step, since the sprayed metal adheres firmly only to the roughened portions and may easily be rubbed off the other portions. On the other hand, if desired, the whole end, or a large portion thereof, may be roughened, as by sand paper, prior to the application of the mask for metal spraying, relying solely on the mask to form the metal areas. It has been found, however, that the preformed roughened areas of the surface of the form, together with the use of the mask during the metal spraying operation doubly insures clearly defined metal areas. When the areas are separated by only narrow spaces it is essential that there should be no rough or overhanging spots of metal.

Referring to Fig. 3, the bare high resistance conductor l8, such as advance, or nichrome wire, is wound on the form by any convenient method, the turns being separated by means of the threaded grooves at I and I which insure against undesired short circuited turns and hold the wire in place during subsequent sliding of a contact arm along the edge of the resistor.

The metal areas deposited by the above spraying process are built up by accretion of minute particles of metal that are retained in place on the roughened surface of the insulation apparently by impaction and/or cohesive attraction.

The strip is finally curled into a circular form,

as shown in Fig. 4, and is adapted to cooperate with a rotating or movable'contact arm'2l that moves along the edge I of the strip opposite to \the edge I having the short circuiting metal areas. Although, of course, the contact arm could be adapted to move along the other edge I, it has been found that the use of the edge 1' shown gives a much smoother mechanical action because of the absence of the shunt areas and the consequent evenness of the wire turns on this edge. This arrangement also gives a more uniform resistance curve for the tapered portion because of the length of wire between the contact arm and the short-circuiting metal areas when the arm is in alignment with the latter.

It has been found desirable in most cases, particularly for volume controls in radio sets to provide a relatively large number of shunts of short length spaced apart a predetermined small distance, the actual dimensions required for a certain taper depending upon several factors such as the size and kind of wire, pitch of the turns, etc. In other cases where smoothness of control is not necessary, the shunts and/or spaces may be made relatively long, each covering a large number of turns thereby giving a more pronounced step effect.

In order to illustrate the tapering effect of various spacings of the metallic shunts, I have listed below the data on several resistor elements constructed in accordance with my invention. The nature or degree of the taper is indicated by the eifective number of turns of wire or percent of turns active in the tapered region remaining after the shunting out of the other turns. The thickness of the shunt area used was of the order of about 0.003", thereby constituting a thin metallic sheet or plate.

Shunt length Spacing mi -ifs c ti ve u lg" 56 )ie 41 r! Ky 36 1 3 4%", at a pitch of 64 turns per inch, giving a total of 270 turns. The tapered portion extended from one end of the resistor about the distance to the other end. The total resistance of the resistor was reduced from 80 ohms to ohms in the above case of the resistor provided with the shunts spaced 3/64, which gives a reduction of 64% in resistance for the tapered portion. In other words, 64% of the turns have been rendered inactive in the tapered portion.

In order to insure that the potential difference between a terminal member 25 and'the first adjacent shunting area l5 shall be uniform in production, regardless of slight variations in location of the terminal member along the resistor, the metal shunting area M has been caused to overlap the terminal 25, being at the same potential therewith. The shunting area I4 is preferably deposited in close proximity to the aperture 23 through which a terminal bolt or rivet (not shown) is adapted to extend. As is well known, such means are employed to secure the resistor element in a casing, and to secure the end terminal member as indicated at 25, in place upon the end turns of the wire and in electrical contact therewith.

It has been found desirable in the manufacture of these resistor elements to wind the wire practically out to the end of the strip and later to pierce the aperture through the strip, thereby cutting off the excess turns of wire by the piercing operation. The length of the area l4, preferably greater than that of areas 15, is such that there is appreciable area thereof extending beyond the side of the terminal lug as shown, regardless of small variations in location of the aperture 23 and the terminal lug 25 during manufacture.

In some constructions that I have designed and built, particularly where it is desirable to have the overall resistance of the potentiometer from one end to the other a predetermined amount, it has been found desirable to deposit a shunting area at the other end of the strip, whereby the distance between the inner edges of these areas is accurate,

\regardless of the usual slight variation in position (of end terminal fastenings. I have illustrated this additional area at I4 adjacent a securing aperture 23 as a modification in Fig. 3, although I have not shown the corresponding aperture in the mask 9 of Figs. 1 and 2 for forming this additional area. The same construction can be used in precision fixed resistors where it is desired to have the overall resistance a predetermined amount.

From the foregoing description it will thus be seen that a manually controlled variable resistor or potentiometer may readily be adapted to provide a desired taper or change in resistance value per unit degree of rotation of the contact element, the means for forming the taper being of permanent nature and particularly adapted to quantity production where uniformity of product is highly desirable. The taper is determined by the width, spacing, and number of the metallic areas. Although I have shown several metallic areas of similar shape, it is obvious that the shapes may be widely varied, particularly in length and that the location of the areas may be changed along the strip, e. g. to the central portion, if desirable. Although it might appear at first glance that the characteristic curve of the above device would be quite uneven, or sharply stepped, it has been found by measurements and by actual practice that the resistance curve is relatively smooth and that it is impossible for the operator of the device, when used in a radio receiving system, to detect by ear, the slightest irregularity in the control of volume.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be re stricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. A variable resistance device comprising a strip support form, a high resistance conductor wound on said form, and means providing spaced low resistance areas along the surface of said form on an edge thereof between said surface and said conductor, said areas being provided by accretive metallic bodies intimately adhering to said surface, for contact with spaced groups of turns of said conductor.

2. A resistance device comprising a supporting strip, a plurality of means located on said strip in closely spaced relation to each otherto provide metallic areas of low resistance, said means being attached to said strip along one edge and overlapping at least a portion thereof, a high resistance conductor helically wound on said strip over said areas, and in electrical contact therewith, whereby turns of said conductor are shunted in groups by said areas.

3. A variable resistance device comprising in combination, a supporting strip, a high resistance conductor wound helically on said strip, a movable contact element adapted to engage said conductor in a path along one edge of said strip, and means comprising accretive low resistance material deposited on another edge of said strip for short circuiting spaced groups of turns of said conductor at points along said strip remote from said path.

4. In a variable resistance device, an elongated insulating strip having notched edges, aplurality of spaced metallic areas impacted in the surface of one of said edges along said strip, a high resistance wire wound helically on said strip in said grooves, groups of turns of said wire being in contact relation with said metallic elements whereby said groups are substantially short circuited thereby, and a movable contact element adapted to contact with said wire along the other of said edges.

5. A resistance device comprising a supporting strip, a plurality of spaced metallic shunting elements located along said strip, a resistance wire extending substantially uniformly along said strip over said areas and in electrical contact therewith, and a terminal member in electrical contact with one of said metallic areas, whereby the potential difference between said terminal member and an adjacent metallic area is substantially independent of slight differences in distances during manufacture between said terminal member and said adjacent area.

6. In a variable resistance device, an elongated insulating strip having sand blasted, roughened areas along one edge thereof, spray deposited, spaced metallic elements providing low resistance areas along said strip on said roughened areas, and a wound high resistance wire located on said strip in intimate contact with said low resistance areas, whereby a predetermined length of said resistance wire is electrically tapered.

7. The herein described method of manufacturing electrical resistors of the variable contact type in which a high resistance element is supported on an insulating strip, which consists in masking portions of said strip leaving spaced unmasked portions, sand-blasting said last named portions, unmasking said strip, spraying low resistance coatings of metal upon said strip over said previously masked and unmasked portions, removing said coatings from said portions previously masked, thereby leaving coated areas of metal on said unmasked portions, and superimposing said resistance element on said strip in contact with said areas.

8. The herein described method of manufacturing tapered variable electrical resistors, which consists in accretion by impacting of electrically separated metallic areas of predetermined size and spacing along an insulating strip. on at least a portion of an edge thereof, winding a plurality of uniformly spaced turns of high resistance conductor on said strip over said areas and in firm physical and electrical contact therewith.

9. A variable resistance device comprising a strip support form, a high resistance conductor wound on said form, and means providing a low resistance area on the surface of said form and extending along an edge thereof between said surface and said conductor, said area being pro-,

said conductor.

FREDERICK L. CREAGER. 

